Apparatus for measuring liquid levels



Dec. 23, 1952 D. R. DE BOISBLANC ETAL APPARATUS FOR MEASURING LIQUID LEVELS Filed June 10, 1949 INVENTORS. L .M OBERLI N DR. DE BoeLANc A T TOPNEVS Patented Dec. 23,l 1952 APPARATUS FOR MEASURING LEQUID LEVELS Desionde R.`fd'loisblanc, Bartlesville, and'lllyman ll/ LOberl Dewey, klal, assignors to Phillips Petroleum Company, a corporation of Delaware Aspiration Jmelc, 1949, serial Ns. oasis '2" claims. (or 73'3o4f) rihis invention relatesto apparatus'formeasluring liquid levels. In'anotheraspect, it relates to such apparatusjin which'the"liquid'levelfisdetermined by variationsincapacitanebetween a pair ofV spaced condenser elementsQ/vthis ca'pacitance change being produced'by changesNin the liquid level.

Heretofore, liquidlevels have been measured by various types ofmsight gauges orflby'float-'controlled indicating devices.V This type of Vn'eafsurement is not suitable "for indicating the Alevel of liquids, such as hydrocarbons', tank cars where severe mechanical shocks'a're encountered uch devices have not proven satisfactoryiorwindicating liquid level of hydrocarbons contained in large tanks, such as bulk storagetanks'oreven in the fuel tanks of airplanesor motorvehicles. Accordingly, in tankcars, it has been customary to utilize slip tubes for measuring'the liquid level. In gauges of'this type, a tube is mountedior movement into and outof 'the liquidfinfthe tank, this tube being vented to th'atms'phere. When the inner end of the tube'is immers'ed'in'tlie liquid, a fine spray issues froni'thevented end thereof whereas, when the innerend of the 'tube visfdisposed in the gas aboveK the liquid, af distinctive white vapor issues from thefvented end ofthe tube. By manipulatingthe'tube, anV operator 4can determine the liquid level within the'tank by noting the slip tube position at which a change in the character of the vaportakes place; V'Iheuse of slipv tube gauges, however, has certain disadvantages. For example, when the tube is rel leased for verticalmovem'ent 'in 'the"'liquid,` the vapor pressure withinthe tankv may expel the tube from its litting with great velocity, which has oftentimes caused injuries tothe joperator' of the gauge. Also, venting of gases or liquids tothe atmosphere produces a fire'haza'rd.v Finally, the slip tube gauges must'be mountedinia'readily iaccessible locationwhich is diflicult to accomplish, particularly in the storageta'nks of' airplanes" or motor vehicles. Y f n It is an object of our invention tol provide'apparatus for measuringa liquid level which is free from the disadvantages encountered by prior systems.

It isa further object to provide apparatusvin which the liquid whose level is" tojbefmeasf ured constitutes a condenser diellecvtri'clwhereby changes in liquid levelvproducefrepresentative changes in capacitance between aset of `spaced condenser elements. I a

It isa further @mean ,providealezvelfmaar which is simple linfconstruction;reliablein opera tion, rugged' and durable.`

Various other objects, advantages and features of the` invention will become apparent to those skilled in the art from a study of the accompanying'di'sclosure and drawings, in which:

Figure 1 is ya vertical sectional View oa DreL ferred embodimentfof the liquid level gauge; l

Figure'Z is ascheinatic circuit diagram' ofthe balancing'circuit; l ,I

Figure 3 is a View illustrating the measurement of theiliquid level; ,V

Figure 4, is a vertical sectional View` oi la modiV iication of the invention; and

Figure 5 is a sectional viewillustrating ateature of'the'inventio'n.

`Referring now'v to 'the drawings in detail, and particularly to Figure l, We have'shown diagrama matic'ally a portion Iii of a tank or container which is filled with a liquid li to'a level l2.' Ajn elongated casing orfrarne I3 is disposed in the tankv and" has its lowerend immersed in the'l lid-r uid l l. Thecasing includes a pair of iiat spaced frame members l, I5 joined by transverse frame members, one of which is shown at i5. The frame members ifi, I5 carry elongated relatively narrow metal strips i8, i9, respectively', and these stripsare mounted in facing closely spaced Darallel arrangement. Frame member i6 is pro-1 vided with openings 2G to admit liquid' to lthe in# terior vof' the casing. Accordingly, the liquidilevel within the casing is always the same as thel liquid level outside the casing. It will be apparent that the strips I8," i9 form a pair of spaced condenser elements, the portion of the strips below liquid level l2 havingthe liquid il as va dielectricmaterial and the portion of the strips aboveliquid level l2 having air as their dielectric material. In'somecaseathe space above liquid level i2 may be iilled with vapors from the liquid Il, but the dielectric constant ofV any such vgases' app-roaches Oneso closely that no appreciable error is introducedrbyassuming-that the region between strips IB, l'above the liquid level il?. is iilled with air. If thev liquid'in the tank is conductive; the strips I8 and'` i9 Vmay be coated with a thin layer of insulating material at the ,surfaces expos'edfto the liquid.* In such case, the term dielectric mate-v rialf still* refers to the 'liquid between the layers ofinsulating material.' a The upper 'ends of metal strips I8, 19ers' connectedV by conductorsl'; 22 toa connector socket 23.1 l

A variable con-denser 24 -is' mounted at the lower end of frame member lll, `the stator and rotor plates of this condenser being completely immersed inthe liquid 'll sothat thisfliquidf constitutesv the entire dielectric material oi" the condenser. The Yshaft of variable vcondenser 24 is 3 connected by a iieXible coupling 25 to a socket 25 at the top of the casing while the rotor and stator of the condenser are connected by leads, not shown, to the socket 23.

The device also includes a control unit 21 which carries an air dielectric variable condenser 28, one end of the condenser shaft being connected to a control knob 29 and the other end of the control shaft being connected by a flexible coupling 30 to plug which is adapted to fit into socket 26. Control unit 21 also has a cable 3I extending therefrom which terminates in a plug 32 mating with socket 23. The unit 21 also contains the circuit components shown by Figure 2 which are connected in an alternating current bridge circuit, as illustrated.

The bridge components include an alternating current source 33, a switch 34, a galvanometer 35, and balancing resistors 36- and 31. Variable condensers 24, 28 are connected in parallel in one Iarm of the bridge circuit, and the capacitance between condenser elements I8, I 9 of Figure 1 is represented by a variable condenser connected in another arm of the bridge circuit.

When the liquid level is below the lower end or metal plates I8 and I9, the capacitance between the plates may be represented by 1LA where 1 is the dielectric constant of the air or other gas between the plates, L is the length of the plates, and A is a factor representing the cross sectional area and spacing of the plates. When the plates are completely immersed in liquid I I, the capacitance between the plates may be represented as K-LA where K is the dielectric constant of liquid II. As the liquid level Varies, the capacitance between elements I8, I9 varies in alinear manner between the two extreme values, LA and KLA, just mentioned. Thus, referring to Figure 3, where the liquid is at a depth D relative to the lower end of the strips I8, I9, the capacitance between plates I8, I9 is:

Variable condensers 24, 28 are connected so that movement of the control knob 29 produces an increase in capacitance of one condenser and a decrease in capacitance of the other condenser. In accordance with the invention, the capacitance of these condensers is so adjusted that their total capacitance, when connected in parallel, varies in the same manner as the capacitance between elements I 8 and I9 upon rotation of control knob 29. Thus, at an angular position 01, condenser 24 has a maximum capacitance equal to KLA and condenser 28 has a minimum capacitance which is negligible. At a second angular position 02, condenser 28 has a maximum capacitance equal to LA while condenser 24 has a minimum capacitance of negligible proportions. Thus, these two settings of the condensers provide capacitances which correspond to the capacitances between elements I8, I9 when the region therebetween is filled with liquid and empty, respectively. As the setting of the condensers is varied, the total capacitance varies in a linear manner, similar to the linear variation produced by a change in the liquid level between condenser elements I8 and I9. At an angular shaft position degrees from position 02, the capacitance of the dual condenser unit may be expressed as follows:

It will be noted that the angle (0a-01) corresponds to the length L of plates I8, I9 while 0, the angle of shaft rotation, corresponds to the term D in the previous relationship. That is, when the capacitance of unit I8, I9 is equal to the capacitance of unit 24 and 28, 0, the angle of shaft rotation, is directly proportional to the liquid level in the tank. Accordingly, with the circuit components connected as shown in Figure 2, when condensers 24, 28 are adjusted by movement of knob 29 so as to produce balance of the bridge, the capacitance of condensers 24, 23 becomes equal to the capacitance of unit I8, I9 with the result that the position of knob 29 gives a direct indication of the depth of liquid in the tank or container I 9.

It is an important feature of the invention that the accuracy of the level indication is not affected by the dielectric constant of the liquid in the tank. This will be apparent from the fact that the angle 0 is equal to the depth D in the aforesaid relationships despite large variations in the value of K, the dielectric constant. That is, condenser 24 compensates for variations in dielectric constant caused by the use of liquids having different dielectric constants in the tank at which the measurement is to be taken. The dielectric constants of liquids stored at various times in a particular tank may vary widely. For example, propane has a dielectric constant of about 2 whereas liquids such as benzene have a much higher dielectric constant in the negihborhood of 12. Accordingly, balancing condenser 24 is very important in that accurate readings may be obtained despite the use of liquids of widely diiering dielectric constant in the same storage tank.

It will be further noted that the function of the bridge circuit is to compare the capacitance of elements I8, I 9 with the combined capacitance of variable condensers 24 and 23. Although both these units in the present example have a linear relationship, in one case between liquid level and capacitance, and in the other case between angle of shaft rotation and capacitance, it is not essential that the relationship be linear, although such relationship is preferred for simplicity. It will be apparent that the control unit 21 is secured to the casing I3 by plugs 30, 32 when a measurement is taken and that the control unit may be readily detached after the measurement is completed. This is very advantageous in that a single control unit may be utilized to take measurements at a number of storage tanks and the delicate galvanometer and other components in control unit 21 are no t subjected to shock and vibration which may be encountered by the casing unit I3, particularly when the casing is mounted in a tank on a railroad car, motor vehicle, or airplane. The casing I3, which is preferably a permanent part of the container, is of very rugged construction and easily withstands shock and vibration.

The accuracy of the results obtained may be increased, if desired, by connecting two condensers 38, 39 in parallel with the unit I8, I9. Condenser 38 is immersed in the liquid whose level is to be measured, and its capacitance is equal to the residual capacity of condenser 24, that is, the capacity between the plates of condenser 24 when it is in its position of minimum capacitance. Condenser 39 has an air dielectric, and its capacity is equal to the residual capacity of condenser 28. The use of these condensers eliminates the small error introduced by assuming that the residual capacity of units 24, 28 is negligible.

A modied form of the invention is shown by Figure 4 in which a casing 42 is formed by elongated supports 43, 44 together with two transverse supports, one of which is-shown-at 45, this lattersupport being provided with openings 546. Thecasing contains a set 41, 48and a set'49, 50 of elongated narrow metal strips, the strips of each set being mounted in facing, closely spaced, parallel relationship. The two sets of stripsare preferably of the same length although this is not essential for proper functioningof the apparatus. The casing 42 is supported in the liquid Whose depth is to be measured by a rod'52 attached by bolts 53 to the top of the casing.- Rod 52 has a graduated scale 54 formed thereon `so that the depth of the casing in the tank may be readily noted.

In operation of the modification Of Figure 4, condenser unit A1, 4t forms one arm of an alterhating current bridge circuit, such as that shown by Figure 2, and condenser unit 49, 50 forms another arm of the bridge circuit. The unit is lowered into the liquid until the bridge circuit is balanced, at which time the capacitance of unit 41, 48 is equal to the capacitance of'unit 49, 55. Accordingly, when balance is obtained, it is known that the liquid level is` at the top end 55 or' plates i1, ri. Thus, if this is not true, and a. portion of the space between these plates is iilled with air, the capacitance oi condenser unit s1, it will be less thanthat of condenser unit 49, Eil.' Accordingly, the depth of liquid may thenbe read upon scale 5ft to a high degree of accuracy. It will be noted that this embodiment is also independent oi the dielectric constant of the liquid in the tank since the same liquid material forms the dielectric of both condenser units.

As stated, with the modication of Figure Ll, the bridge circuit is balanced when the liquid level reaches point 55. If the liquid level is lower, it is diicult to obtain ac-curate readings although the bridge circuit may be adjusted for balance when a predetermined relationship exists between the capacitance of the condenser units, rather than when the capacitances are equal. It is, of course, impossible with the modification of Figure 4 to determine the depth if the liquid level is below plates ii and 4S. This difficulty may be overcome by utilizing the apparatus of Figure 5 in which two casing sections 51, 58v are connected by a hinge 53. Casing section 51 carries the plates di, it and casing section 53 carries plates lie and 59. The two casing sections are preferably biased to a position in which one section is perpendicular to the other by a spring 5t and plates da, 5i! are always immersed in the liquid whose depth is to be measured. With this modiiication, readings may be taken accurately where the liquid level is quite low, as compared with Figure 4, since the p-lates d1, t3 may be positioned much closer to the bottom of the tank. A similar result may be obtained with the apparatus of Figure 1 by mounting condenser 24 at a position 5l on support it. In this fashion, the lower end of plates I8, I@ may be extended to the bottom of the tank.

it will be apparent that We have provided a liquid level indicator utilizing the dielectric constant of the liquid whose level is to be measured as a means for determining its depth. The accuracy of the reading is not affected by the dielectric constant of the liquid and no calibration thereof is required when liquids of differing dielectric constant are stored at diiferent times in a single tank. The capacitance measuring unit is well adapted for permanent mounting in a tank and these permanently mounted parts may be made rugged and durable enough as to With- 6 stand considerable abuse in the viield, a separate housing being provided for the galvanometer and other parts of the bridge circuit which is readily detachable from the parts permanently aixed to the liquid container.

While the invention has been described in connection with present, preferred embodiments thereof, it is to be understood that this description is illustrative only and is not intended to limit the invention, the scope of which is deiined by the appendedI claims.

Having described our invention, we claim: l

1. Apparatus for measuring a liquid level comprising, in combination, a pair of spaced condenser elements, -the liquid Whose-level ris to be measured constitutingV aportion of the condenser dielectric material, anda gas constituting the remainder or" said dielectric material, whereby variations in liquid level change the `relative proportions of gas and liquid dielectric material, a variable condenser having said liquid as ardielectric material, a second Variable condenser having said gas as a dielectric material, said variable condensers being mechanically interconnected so that an increase in capacitycf one variable condenser produces a decreasein capacityof the other variable condenser, and means for comparing the capacitance of said condenser elements with the combined capacitance of said Variable condensers.

2. Apparatus for measuring a liquid level comprising, in combination, a pair of spaced condenser elements, the liquid whose level is to be measured constituting a portion of the condenser ielectric material, and a gas constituting the remainder or said dielectric material, whereby variations in liquid level change the relative proportions of gas and liquid'dielectric material with a corresponding change in the capacitance between said elements, means for producing a balancing capacita-nce which varies in a manner corresponding to that of said condenser elements including a variable condenser having said liquid as a dielectric material, and a second variable condenser having said gas as a dielectric material, said variable condensers being mechanically interconnected so that an increase in capa-city of one variable condenser produces a decrease in capacity of the other variable condenser, and

means for comparing the capacitance of said condenser elements with the combined capacitance of said variable condensers.

3. Apparatus for measuring a liquid level comprising, in combination, a pair of spaced condenser elements, the liquid whose level is to be measured constituting a portion of the condenser dielectric material, and a gas constituting the remainder oi said dielectric material, whereby variations in liquid level change the relative proportions of gas and liquid dielectric material with a corresponding change in the capacitance between said elements, means for producing a balancing capacitance which varies in a manner corresponding to that of said Condenser elements including a variable condenser having said liquid as a dielectric material, and a second variable condenser having said gas as a dielectric material, said variable condensers being mechanically interconnected so that an increase in capacity of one variable condenser produces a decrease in capacity of the other variable condenser, the effective plate area and spacing of each variable condenser being equal to that of said condenser elements when the other variable condenser is at a position of minimum capacitance, and means for comparing the capacitance of said condenser elements with the combined capacitance of said variable condensers.

4. Apparatus in accordance with claim 3 in which the comparing means consists of an alternating current Wheatstone bridge circuit, said bridge including said spaced condenser elements as one arm thereof, and another arm of said bridge including both of said variable condensers connected in parallel circuit arrangement.

5. Apparatus for measuring a liquid level comprising, in combination, an elongated casing" adapted for immersion in a container containing a liquid whose level is to be measured, a pair of elongated relatively narrow strips mounted within said casing in closely spaced parallel arrangement, said casing being perforated to allow said liquid to enter said casing, means supported by said casing for making electrical connections to the respective metal plates, a variable condenser mounted on said casing, said condenser having its stator and rotor plates immersed in said liquid, and a exible coupling carried by said casing to move the rotor of said variable condenser.

6. Apparatus for measuring a liquid level comprising, in combination, an elongated casing adapted for immersion in a container containing a liquid whose level is to be measured, a pair of elongated relatively narrow strips mounted within said casing in closely spaced parallel arrangement, said casing being perforated to allow said liquid to enter said casing, means supported by said casing for making electrical connections to the respective metal plates, a variable condenser mounted on said casing, said condenser having its stator and rotor plates immersed in said liquid, and a flexible coupling carried by said casing to move the rotor of said variable condenser, a unit including a support, a second variable condenser mounted on said support, means for rotating said second variable condenser, a flexible cable connecting the shaft of said second condenser to the shaft of said flexible coupling so that the capacitance of one variable condenser increases as that of the other condenser is decreased, and circuit components mounted in said support and connected in an alternating current Wheatstone bridge circuit, said circuit including said spaced condenser elements as one arm thereof, and said variable condensers as another arm thereof, said variable condensers being connected in parallel circuit arrangement.

7. Apparatus for measuring a liquid level comprising, in combination, a pair of spaced condenser elements, the liquid whose level is to be measured constituting a portion of the condenser dielectric material, and a gas constituting the remainder of said dielectric material, whereby variations in liquid level change the relative proportions of gas and liquid dielectric material with a corresponding change in the capacitance between said elements, means for producing a balancing capacitance which varies in a manner corresponding to that of said condenser elements including a variable condenser having said liquid as a dielectric material, and a second variable condenser having said gas as a dielectric material, said variable condensers being mechanically interconnected so that an increase in capacity of one variable condenser produces a decrease in capacity of the other variable condenser, the effective plate area and spacing of each variable condenser being equal to that of said condenser elements when the other variable condneser is at a position of minimum capacitance, means for comparing the capacitance of said condenser elements with the combined capacitance of said variable condensers, and a pair of fixed condensers connected in parallel with said condenser elements, one xed condenser being immersed in said liquid and having a capacitance equal to the residual capacity of said rst variable condenser, the other fixed condenser being disposed in said gas and having a capacitance equal to the residual capacity of said second variable condenser.

DESLONDE R. DE BOISBLANC. LYMAN M. OBERLIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,357,023 Reid et al Aug. 29, 1944 2,511,398 De Giers et al June 13, 1950 FOREIGN PATENTS Number Country Date 441,576 Great Britain Jan. 22, 1936 114,980 Sweden Sept. 18, 1945 

